There is known a technique of measuring a change in a frequency of a crystal oscillator by applying light from a light source to the crystal oscillator and measuring the intensity of the reflected light caused by the vibration of the crystal oscillator.                [Patent Document 1] Japanese Laid-open Patent Publication No. 08-15122        [Patent Document 2] Japanese Laid-open Patent Publication No. 08-327398        
However, it is difficult to detect a state of the crystal oscillator before a transition to an output stop state (for example, clock stop) based on such a measurement result of the change in the frequency of the crystal oscillator that is obtained according to the conventional technique as described above. It is noted that the output stop of the crystal oscillator may occur suddenly due to abnormality of the crystal oscillator or the like.